Polyolefin having multimodal molecular weight distribution has at least two components each having different molecular weight. For example, polyolefin contains a high molecular weight component and a low molecular weight component in relatively proper proportions. Many studies have been conducted for the preparation of a polyolefin having broad molecular weight distribution or multimodal molecular weight distribution. One method among them is a post-reactor process or a melting blending process in which polyolefin having at least two different molecular weights are blended before or during the processing of the polyolefin. For example, U.S. Pat. No. 4,461,873 discloses a blending method of physically blending two different kinds of polymers for preparing a bimodal polymer blend. When such a physical blending method is used, it is liable to produce a molded form having high gel component, a product appearance is deteriorated owing to the gel component, and thus the polyolefin cannot be used for the films. Further, the physical blending method requires a complete uniformity, so there is a disadvantage of the preparing cost being increased.
Another method for preparing polyolefin having multimodal molecular weight distribution, for example bimodal molecular weight distribution is to use a multistage reactor which includes two or more reactors. In the multistage reactor, a first polymer component having one molecular weight distribution among two different molecular weight distribution of the bimodal polymer, is prepared in a certain condition at a first reactor, the first polymer component prepared is transferred to a second reactor, and then a second polymer component having different molecular weight distribution from that of the first polymer component, is prepared in a different condition from that of the first reactor, at the second reactor. The above-mentioned method solves the problems relating to the gel component, but it uses the multistage reactor, so the production efficiency may be decreased or the production cost may be increased. Also, when the high molecular weight components are prepared in the first reactor, the low molecular weight components are not prepared in the second reactor and thus the finally manufactured polyolefin particles may be made only by the high molecular weight components.
Still another method for preparing polyolefin having broad molecular weight distribution or multimodal molecular weight distribution is to polymerize the polyolefin by using a mixture of catalysts in a single reactor. Recently, in the pertinent art, the various attempts have been made for producing polyolefin having broad molecular weight distribution or multimodal molecular weight distribution, by using two or more different catalysts in a single reactor. In this method, the resin particles are uniformly mixed in a level of sub-particles, thus the resin components each having different molecular weight distribution exists in a single phase. For example, U.S. Pat. Nos. 4,530,914 and 4,935,474 disclose a method for preparing polyolefin having broad molecular weight distribution by polymerizing ethylene or more higher alpha-olefins in the presence of a catalyst system comprising two or more metallocenes each having different propagation and termination rate constants and alumoxane. Further, U.S. Pat. Nos. 6,841,631 and 6,894,128 disclose a method for preparing polyethylene having bimodal or multimodal molecular weight distribution by using a metallocene-type catalyst comprising at least two metal compounds and the usage of the polyethylene for manufacturing films, pipes, hollow molded articles and so on. Polyethylene produced in this way has a good processability, but the dispersed state of the polyethylene component in the molecular weight per unit particle is not uniform, so there are disadvantages of rough appearance and unstable physical properties even in relatively good processing conditions.
U.S. Pat. No. 4,937,299 discloses a method for preparing polyolefin by using a catalyst system comprising at least two kinds of metallocenes each having different reactivity ratio with respect to monomer to be polymerized. U.S. Pat. No. 4,808,561 discloses a method for preparing olefin polymerization supported catalyst by reacting metallocene with alumoxane in the presence of a carrier. The metallocene is supported in the carrier to form solid power catalyst. As the carrier, inorganic oxide materials such as silica, alumina, silica-alumina, magnesia, titania, zirconia and the mixture thereof, and resinous materials such as polyolefin (for example, finely divided polyethylene) can be employed, and the metallocenes and alumoxanes are deposited on the dehydrated carrier material.
U.S. Pat. No. 5,539,076 discloses a mixture catalyst system of metallocene/non-metallocene for preparing a specific bimodal high-density copolymer. The catalyst system is supported by an inorganic carrier. The carrier such as silica, alumina, magnesium-chloride and the mixture catalyst of Zeigier-Natta and metallocene are disclosed in U.S. Pat. No. 5,183,867, European publication No. 0676418A1, European Patent No. 0717755B1, U.S. Pat. No. 5,747,405, European publication No. 0705848A2, U.S. Pat. No. 4,659,685, U.S. Pat. No. 5,395,810, European publication No 0747402A1, U.S. Pat. No. 5,266,544 and WO 9613532A1 etc. The mixture catalyst of Zeigier-Natta and metallocene supported has relatively low activity than single uniform catalyst, so it is difficult to prepare polyolefin having properties suitable for a specific use. In addition, since polyolefin is prepared in a single reactor, the gel which is generated in the blending method may be produced, it is difficult to insert comonomer to high molecular weight components part, the form of polymer produced may be poor and further two polymer components may not be uniformly mixed, so the quality control of the produced polyolefin may be difficult.
Korean Patent No. 1132180 discloses a more than one metallocene mixture catalyst system for preparing a multimodal polyolefin copolymer. The catalyst system has a disadvantage that an amount of comonomer of low molecular weight portion of bimodal polymer is not low. In order to satisfy the mechanical strength of polymer and long-term water resistance characteristic as a pipe, the low molecular weight portion should be low the amount of the comonomer, and the high molecular weight portion should be high the amount of the comonomer. However, in the catalyst system, the amount of comonomer of the high molecular weight portion is high, but long-term water resistance characteristic may be reduced due to the comonomer included in the low molecular weight portion. Also, in the catalyst system, since a first metallocene for preparing the low molecular weight polymer has a low hydrogen reactivity, a control of the molecular weight with maintaining appropriate bimodal may difficult, and since molecular weight of two metallocene compounds is too high, melt flow index (MIE, 2.16 kg/10 mins) is too low, and since melt flow index ratio is too broad, mechanical properties of molded form is lowered.
U.S. Pat. No. 5,594,078 discloses a catalyst composition consisting of bridged fluorenyl-containing metallocene, non-bridged metallocene and co-catalyst, and method for producing olefin polymer by using the catalyst composition. However, the U.S. Pat. No. 5,594,078 only discloses simple information such as melt flow index, molecular weight distribution of the polymer obtained by batch-type polymerization, but physical properties of polyolefin such as a moldability, mechanical properties, an appearance of the molded form, which are required industrially, are not taken into account. Also, U.S. Pat. No. 7,619,047 discloses at least two metallocene mixture catalyst system for preparing a multimodal copolymer. The first metallocene used in the mixture catalyst system of U.S. Pat. No. 7,619,047 includes a linear alkenyl group as a substituent in bridged cyclopentadienyl groups, which is different from the bridged fluorenyl-containing metallocene disclosed in U.S. Pat. No. 5,594,078. However, the U.S. Pat. No. 7,619,047 only discloses a melt flow index and molecular weight of the prepared polyolefin, physical properties of polyolefin such as a moldability, mechanical properties, an appearance of the molded form which are required industrially are not taken into account.